The proposal research is directed to uncovering the mechanism by which ligand (agonist) occupation is coupled to the activation and desensitization processes in the nicotinic cholinergic receptor. The coupling step is the distinguishing feature in the actions of agonists, partial agonists, metaphilic antagonists and classical antagonists. Quantitation of ligand occupation simultaneous with activation has been achieved with a clonal muscle cell, BC3H-1. Receptor occupation is quantitated from ligand inhibition if the initial rate cobra alpha toxin binding and activation of the channel monitored through 22Na ion permeability. With these measurements, we propose to analyze the state function for receptor activation and desensitization from the binding isotherms following initial and equilibrium ligand exposure. The cell culture system should be amenable to genetic manipulation and we hope to characterize mutant receptors with altered activation and desensitization parameters. Finally, the BC3H-1 cells elaborate alpha and beta-adrenergic receptors and a similar approach of correlating ligand occupation with receptor activation (ion permeability) is proposed for the adrenergic receptor.